The present invention relates to a semiconductor device of a hetero-junction bipolar transistor structure, and a method for fabricating the same.
Recently optical communications systems and mobile communication systems which have high efficiency are required. To make these systems highly efficient semiconductor devices are essential. Hetero-junction bipolar transistors (hereinafter called xe2x80x9cHBTsxe2x80x9d), which are known as high-speed devices, are one of such devices whose efficiency improvement is prospective.
A structure of a conventional HBT will be explained with reference to FIG. 11.
A collector contact layer 102 formed of an n+-InGaAs layer is formed on a semi-insulating InP substrate 100. A collector layer 104 of an i-InGaAs layer is formed on the collector contact layer 102. A base layer 106 of a p+-InGaAs layer is formed on the collector layer 104. An emitter layer 108 of an n-InP layer is formed on the base layer 106. An emitter contact layer 110 of an n+-InGaAs layer is formed on the emitter layer 108. An emitter electrode 112 of WSi film is formed on the emitter contact layer 110. The emitter contact layer 110 and the emitter layer 108 are processed in a mesa-shape, and a base electrode 116 is formed on an exposed part of the base layer 106. The base layer 106 and the collector layer 104 are processed in a mesa-shape, and a collector electrode 118 is formed on an exposed part of the collector contact layer 102. Thus, an InP/InGaAs-based HBT is formed.
To make the HBT-ICs more speedy, a higher maximum oscillation frequency fmax is necessary. A maximum oscillation frequency fmax is expressed by
fmax=(fT/(8xcfx80xc3x97RBxc3x97CBC))
wherein a maximum cut-off frequency is represented by fT, a base resistance is represented by RB, and a base-collector capacitance is represented by CBC. A maximum oscillation frequency fmax is proportional to a reciprocal of a square root of a base resistance RB ((1/(RB))), and for a higher maximum oscillation frequency fmax, it is necessary to obtain a lower base resistance RB.
In GaAs-based HBTs, recently carbon (C) is dominantly used as a dopant for the bases from the viewpoint of ensured reliability, etc., and doping techniques for higher concentrations of above 1xc3x971020 cmxe2x88x923 have been developed.
On the other hands, in InP/InGaAs-based HBTs, actually doping techniques using carbon as a dopant for the base have not been sufficiently established. The base layer cannot be heavily doped with carbon, and this will be because carbon is not dissociated from hydrogen in forming InGaAs layer to be the base layer to be taken in the films in the form of CH, and the carbon does not function as an acceptor (hydrogen passivation). This phenomenon is conspicuous especially in MOCVD method using hydrogen as a carrier gas and a hydrogen content gas as a source gas.
As a result, InP/InGaAs-based HBTs have very high maximum cut-off frequencies fT but cannot have sufficiently maximum oscillation frequencies fmax.
An object of the present invention is to provide a structure of a semiconductor device which enables an InP/InGaAs-based HBT to have a lower base resistance, and a method for fabricating the same.
The above-described object can be achieved by a semiconductor device comprising: a collector layer; a base layer of a carbon-doped GaxIn1xe2x88x92xAsySb1xe2x88x92y layer having one surface connected to the collector layer; an emitter layer connected the other surface of the base layer; a base contact layer of a carbon-doped GaAsSb layer electrically connected to the base layer; and a base electrode formed on the base contact layer. The semiconductor device of such structure can have a much reduced base resistance RB, whereby InP/GaInAsSb-based HBTs including InP/InGaAs-based HBTs can have higher maximum oscillation frequency fmax. Because of the carbon-doped semiconductor layer the semiconductor device can have higher reliability.
In the above-described semiconductor device, it is preferable that the base contact layer is formed on said one surface or the other surface of the base layer.
In the above-described semiconductor device, it is possible that the base contact layer is formed on a surface of the collector layer connected to the base layer and has a side surface connected to a side surface of the base layer.
In the above-described semiconductor device, it is possible that the base contact layer is formed on a surface of the emitter layer connected to the base layer and has a side surface connected to a side surface of the base layer.
In the above-described semiconductor device, it is possible that the device further comprises a surface passivation layer for protecting the base contact layer formed on the surface of the base contact layer with the base electrode formed on. Because of the surface passivation layer covering the surface of the base contact layer, surface recombination on the base contact layer can be restrained, whereby dependence of current gains on sizes can be restrained, and the semiconductor device can have higher reliability.
In the above-described semiconductor device, it is possible that the base contact layer is formed of a carbon-doped GaInAsSb layer in place of said carbon-doped GaAsSb layer.
In the above-described semiconductor device, it is possible that an As composition y of the GaxIn1xe2x88x92xAsySb1xe2x88x92y is 1, so that the base layer is formed of a InGaAs layer.
In the above-described semiconductor device, it is preferable that an In composition x of the GaxIn1xe2x88x92xAsySb1xe2x88x92y is 0, so that the base layer is formed of a GaAsSb layer.
In the above-described semiconductor device, it is preferable that a dopant concentration of the base contact layer is not less than 1xc3x971020 cmxe2x88x923.
The above-described object can be also achieved by a method for fabricating a semiconductor device comprising the steps of: forming a first semiconductor layer on a semiconductor substrate; forming a base layer of a carbon-doped GaxIn1xe2x88x92xAsySb1xe2x88x92y layer on the first semiconductor layer; forming a second semiconductor layer on the base layer; patterning the second semiconductor layer in a mesa-shape; forming a base contact layer on the base layer exposed by patterning the second semiconductor layer; and forming a base electrode on the base contact layer. By fabricating the above-described semiconductor device fabricating method, the semiconductor device can have a much reduced base resistance RB, whereby InP/GaInAsSb-based HBTs including InP/InGaAs-based HBTs can have higher maximum oscillation frequency fmax. Because of the carbon-doped semiconductor layer the semiconductor device can have higher reliability.
In the above-described method for fabricating a semiconductor device, it is preferable that the method further comprises, after the step of patterning the second semiconductor layer, a step of removing the base contact layer in a exposed region which is exposed by patterning the second semiconductor layer, wherein in the step of forming the base contact layer, the base contact layer having a side surface connected to the base layer is formed on the first semiconductor layer exposed by removing the base layer.
In the above-described method for fabricating a semiconductor device, it is preferable that in the step of forming the base layer, the base layer of an InGaAs layer which corresponds to the GaxIn1xe2x88x92xAsySb1xe2x88x92y layer whose As composition y is 1, or a GaAsSb layer which corresponds to the GaxIn1xe2x88x92xAsySb1xe2x88x92y layer whose In composition X is 0 is formed.
In the above-described method for fabricating a semiconductor device, it is preferable that in the step of forming the base contact layer, the base contact layer is formed of a material which lattice-matches with a material forming the base layer. The base contact layer is formed of a material which lattice-matches with a material forming the base layer, whereby characteristic deterioration of the semiconductor device due to lattice deformation can be prevented.
In the above-described method for fabricating a semiconductor device, it is preferable that in the step of forming the base contact layer, the base contact layer is formed of a carbon-doped GaAsSb layer or a carbon-doped GaInAsSb layer. The base contact layer is formed of such carbon-doped materials, whereby the base contact layer can effectively have a low resistance, and the semiconductor device can have higher reliability.
In the above-described method for fabricating a semiconductor device, it is preferable that the method further comprises, before the step of forming the base contact layer, a step of thermal-treating for eliminating hydrogen in the base layer. Hydrogen in the base layer is eliminated, whereby carbon bonded with the hydrogen is electrically activated, whereby the base layer can have a further lower resistance.
In the above-described method for fabricating a semiconductor device, it is preferable that the method further comprises, after the step of patterning the second semiconductor layer, a step of forming a sidewall insulation film on a side wall of a mesa of the second semiconductor layer.
In the above-described method for fabricating a semiconductor device, it is preferable that the method further comprises, after the step of forming the base contact layer, a step of forming a surface passivation layer on the base contact layer for protecting the base contact layer. Because of the surface passivation layer covering the surface of the base contact layer, surface recombination on the base contact layer can be restrained, whereby dependence of current gains on sizes can be restrained, and the semiconductor device can have higher reliability.
In the above-described method for fabricating a semiconductor device, it is preferable that the first semiconductor layer or the second semiconductor layer is an emitter layer of an InP layer.
The structure of the semiconductor device according to the present invention is applicable to not only a semiconductor device including a collector layer, a base layer and an emitter layer sequentially deposited on a semiconductor substrate, but also a semiconductor device of the so-called collector-up structure including an emitter layer, a base layer and a collector layer sequentially deposited on a semiconductor substrate.